We include products we think are useful for our readers. If you buy through links on this page, we may earn a small commission. Here’s our process.
If the leaves of poison oak are broken or the plant is damaged in some way, it releases an oil – urushiol – that is poisonous to humans. The oil triggers the body’s immune system and produces a rash.
Even dead leaves, stems, or roots contain the oil; it can also be inhaled if the plants are burned. Urushiol is the same oil that is produced by poison ivy and sumac.
Alternatively, indirect contact can occur if the oil gets onto clothes or is carried on a pet’s coat. Although cats and dogs are not generally affected by urushiol, they can carry it.
Most people, but not all, show an allergy to the oil, referred to as allergic contact dermatitis. It usually appears between half a day and 3 days after contact with the plant oil.
People who are allergic to the oil do not have a reaction until they have a second contact with the oil. The immune system learns to recognize the oil from the first occasion and then reacts to it aggressively on future contact.
According to the Centers for Disease Control and Prevention (CDC), most people are sensitive to the oil in poison oak, ivy, and sumac:
“When exposed to 50 micrograms of [the oil], an amount that is less than one grain of table salt, 80 to 90 percent of adults will develop a rash.”
A poison oak rash appears where the contact with the oil occurred. However, it can also form on parts of the body not contacted by the plant.
It normally starts as itching and mild irritation and gradually worsens developing in to a red rash that gradually gets more itchy.
Bumps will form, which can turn into blisters.
The rash gradually resolves over a period of 3-4 weeks.
The irritation varies depending on which area is affected, but some features are common anywhere on the body:
- It always involves intensely itchy, red skin.
- Often there are multiple streaks in the area of skin brushed by the oil.
- Swelling is common, sometimes hives can appear.
If there is a large area affected by the rash, or it is in a place that makes movement difficult, the problem is more serious. This is most commonly the case for people who are often exposed to the plants, such as those working where poison oak grows.
The rash should quickly settle down and begin healing; but it can take a few weeks for a poison oak rash to clear up fully.
The rash itself cannot be spread between people. However, anyone who is regularly exposed should be careful to avoid spreading the oil and causing a reaction in other people; for instance, if the poison oak oil is transferred from protective gear, clothes, and tools.
Any swelling beyond small hives in the affected area should visit a doctor. For anyone who has a wider reaction, it is important to get medical help.
People should call for immediate medical help or go to an emergency room straight away if any reaction causes these signs of severe allergy:
Recognizing the actual plant itself is tricky because individual poison oaks come in different forms. Below are some examples:
If a person thinks they have come into contact with poison oak, they should wash their skin and clothes in warm, soapy water as soon as possible.
The allergic rash should start to settle down naturally. Ways to soothe the rash and prevent it from getting worse include:
- Staying away from the poison oak and keeping the rash from getting irritated or infected by anything else.
- Not scratching at the itch as this can cause more damage, itchiness, pain, and infection.
- Simple, clean, warm water bathing, followed by clean, dry, gentle patting or natural drying helps.
- Using emollient cream to protect the skin from losing moisture and keep the skin’s barrier function. Emollient cream is available for purchase online.
Antihistamines, like diphenhydramine, can ease the itchiness. A range of antihistamines is available for purchase over the counter or online.
However, it can also make people drowsy, so it should not be taken before operating heavy machinery or driving.
People should see a doctor if:
- the rash does not go away within 3 to 4 weeks
- symptoms get worse
A doctor can prescribe oral or topical steroids or stronger prescription antihistamines.
The easiest advice for avoiding poison oak and the nasty rash it can cause is to:
- Know where the plants are and avoid them.
- Know that the oil is released by damage to the plant.
- For unknown plants, look out for the leaf-group pattern of poison oak.
- If a pet is thought to have contacted poison oak, wash it thoroughly.
- Do not burn poison oak as the oil can be inhaled and cause internal damage.
People whose work puts them at a higher risk of contact with poison oak oil need to take measures to protect themselves. Employers must also help.
The National Institute for Occupational Safety and Health (NIOSH) have detailed some good tips for workers with a fact sheet on poisonous plants.
The following video from NIOSH shows why the skin reacts to poisons that cause allergy. NIOSH have used animated drawings to simplify the way the immune cells of the body react to poison oak and produce the rash: